A microscope is an optical instrument that has historically employed two or more lenses to make enlarged images of minute objects. In the so-called compound microscope, an objective lens is placed near a target to be viewed and the user views an image of the target through an eyepiece lens. In many installations, the eyepiece lens is replaced with a camera, often a digital camera using a charge coupled device as an imaging element.
Microscopes are commonly used to inspect parts in industrial manufacturing operations. To use a manually focused microscope, the operator typically adjusts the position of the entire microscope along an axis defined by the center of the objective lens and the eyepiece lens, changing distance between the objective lens and the object, thus adjusting the microscope's region of best focus to place it on the target surface to be imaged. Alternatively, the target surface may be moved axially to place the target surface at the microscope's region of best focus. The lens or target surface is typically moved using a precision mechanism. In industrial inspection applications, the microscope may be automatically focused by the use of an electronic focus sensing system. The focus sensing system produces an electronic output signal that is used to command a precision servomechanism to maintain the target within the best focus region. In one class of microscope, an “infinity corrected” objective lens may be moved independently of the eyepiece to focus the microscope. Infinity corrected objective lenses are especially well suited to servomechanism controlled focus applications, because the servomechanism need only move the objective lens, rather than the target or the entire microscope.
Prior art devices often project a spot of light through the microscope objective onto a target surface and collect returning light to generate a focus error signal. Some prior art devices sense focus by projecting a line of light onto the target surface. In astigmatic systems, the projected line is created by placing a toric lens between the light source and the objective lens. A toric lens has at least one non-axially symmetric surface formed by a section of a torus, and in the limit where one radius of curvature becomes infinite, is a cylinder lens. Light reflected from the target surface returns through the objective lens and is diverted to a focus sensing module that generates an electric focus output signal. In this type of system, the focus output signal represents an average of the target surface distance sensed along the line, also called “line averaging.” Such systems typically include a servomechanism to maintain the focus automatically by driving the precision mechanism to displace the target surface or the objective lens. It should be noted that this method of projecting a focused line is often not itself an integral part of the method of focus sensing, but only a way of extending the sensed region to provide an averaged focus error signal. Various methods of focus sensing may be used with this line averaging technique.
However, line averaging inefficiently uses light because the focus line typically diverges strongly after reflection, thus reducing the amount of light collected by the objective lens. This technique is particularly inefficient on specular surfaces, meaning mirror-like surfaces. Also, aberrations, particularly astigmatism aberrations, can cause wave front errors that degrade or bias the focus output signal. Moreover, because the amount of astigmatism introduced by the toric lens is fixed, the user cannot choose a length of line that is appropriate to the specific application, nor can computer-based image processing systems direct the focus sensing spot to a specific location.
Many common optical systems, such as laser printers, include scanning systems that effectively form a line by scanning a spot of light over a surface at a relatively high speed. These systems often generate the line by employing a multi-faceted rotating mirror. However, this type of system is relatively complex, is often expensive, and is prone to measurement errors caused by artifacts introduced by the rotating mirror. Furthermore, a precision multi-faceted mirror scanner is generally relatively expensive and difficult to produce. Polygon scanning systems have typically not been used in focus sensing systems for reasons of cost, complexity and size.
Focus sensing systems using the techniques described above can only sense focus along a single, nominally straight line. Astigmatic systems generate an average focus output signal based upon the axial distance from the objective lens to the target surface along the line, and intrinsically average over the line. However, irregular target surfaces often may benefit from an average focus signal be generated over a two-dimensional area, or require that the location of the sample point on the target surface for generating the focus signal be randomly selectable.
Accordingly, there is a need for a distance measuring system that is capable of generating focus error information from: 1) an average of distance from an objective lens to a target surface sensed along a selectable line on the target surface, 2) an average of distance information sensed over an area defined on the target surface, or 3) distance information sensed from a selectable location on the target surface. This distance measuring system should be capable of being installed on a microscope without substantially interfering with the microscope's viewing and inspection properties. The system should be inexpensive and should generate relatively little vibration or other scan-related artifacts.